The present invention is directed to a method for making a centrifuge tube and, more particularly, is directed to a unique approach for making thin wall centrifuge tubes having an improved polymer structure to provide desirable structural advantages.
Although present methods for making centrifuge tubes have been generally satisfactory, the need for making better centrifuge tubes with greater integrity becomes more acute as the sophistication of centrifugation, especially ultracentrifugation, increases. The stresses which are now placed upon centrifuge tubes in the ultracentrifugation field are extreme, requiring the properties of the centrifuge tube to be as nearly perfect as possible.
A typical present method for the manufacture of thin wall centrifuge tubes utilizes the injection molding process which produces a very satisfactory tube, but unfortunately has some undesirable characteristics. One particular disadvantage with respect to injection molded centrifuge tubes is the fact that the thin wall has a non-uniform thickness which is the result of slight movement of the core pin utilized during the process. Very often the centrifuge tube has a thin region extending from the bottom of the tube to the top with the thinnest region near the bottom sidewall. After cooling, the injection molded tube must be removed from the core pin requiring an undesirable slight tapered shape to the centrifuge tube in order to permit extraction from the core pin.
In the injection molding process the mold itself is relatively cold compared to the temperature of the thermoplastic material entering the mold. The injection molded tube, consequently, has a weak region caused by the two moving cold fronts bonding at a thin region called a knit-line. The deflection of the core pin also contributes to the development of the knit-line. Unfortunately, in some circumstances this knit-line is a weak point in the tube and, in some instances, will fail by cracking during high speed centrifugation.
An injection molded tube has a gate region of non-uniform polymer structure at the tube's hemispherical bottom which is caused by the rapidly changing temperatures as the thermoplastic material of high temperature enters through a gate to the relatively cool mold. Also, the shear of the plastic melt, as well as changes in the direction of the plastic flow, adversely affect this gate region on the bottom of the tube.
Because the thin wall tube cavity in the mold is very small and narrow, high injection pressures and speeds are required to move the molten material through the tube mold before the material cools. The material entering the cool mold at high pressure and temperature causes high residual stresses in the tube which, coupled with centrifugally induced forces during centrifugation with the finished tube, may effect the integrity of the tube. The overall mechanical properties of the injection molded centrifuge tube are somewhat affected by the high plastic melt temperature that is required for the molding. In many instances, the tube tends to be somewhat brittle and the tube, although somewhat strong in the axial direction, is weak in the circumferential direction.
In the injection molding process typically there is a longer cycle time required for the mold because of the cooling step of the process which occurs after the injection of material into the mold. The high temperatures and pressures used in the injection molding process for making a thin wall centrifuge tube approach the degradation limit of the polymer by the combined effects of the thermal and sheared degradation.
Another process utilized in the making of thin wall centrifuge tubes is extrusion blow molding, but there are certain inherent characteristics which occur during this process that are not desirable. Thin wall tubes molded by the extrusion below molding process typically may have die marks or striations coincident with the centerline axis of the tube caused by the extrusion process with stretching during the blowing process. In the extrusion blow molded process a pinch-off is required of the extruded cylindrical molten preform for sealing during the blowing process. The pinch-off occurs at the bottom of the centifuge tube and is a potential weak region because of the welding process. Also, there is the requirement that a tail or excess material portion at the bottom of the tube at the pinch-off be removed from an extrusion blow molded tube so that the tube will have a good fit when placed in the centrifuge rotor.
Although some containers, such as decorative bottles, are made using a combination of injection molding and blow molding, this process has not been considered or used for making thin walled centrifuge tubes because this process has been regarded as too unstable with poor thermal uniformity to make an acceptable thin wall configuration. The general rules applied to making bottles with injection blow molding would be violated by making thin wall centrifuge tubes. The final thickness of the bottle configurations made by this process are considerably thicker than the thickness of the preform needed to make a thin wall centrifuge tube. Therefore, the final thickness of the thin wall centrifuge tube is an order of magnitude thinner than the bottle wall thickness. It is a general rule in injection/blow molding that the minimum parison thickness be at least 0.080 inches and the minimum mold core pin diameter be not less than 0.188 inches. The process for making a thin wall centrifuge tube requires the parison thickness as well as the core pin diameter to be considerably less than the above general minimum dimensions. Also, the ratio of the core pin length to the core pin diameter (L/D) should never be more than twelve to one. However, to make a thin wall centrifuge tube, the L/D ratio of the core pin is required to be greater than twelve to one. Consequently, the general known capabilities of injection blow molding taught away from the idea of using the process to make not only very thin walled containers, but also thin walled centrifuge tubes that must be perfectly made to withstand the extremely high centrifugally induced forces.